Method for injecting a substance into a boiler of a garbage incineration plant

ABSTRACT

A method is provided for injecting a substance into a boiler of a garbage incineration plant using a gaseous propellant, the substance being conducted, starting from at least one distributor, via lines which branch off from the distributor, in each case to a nozzle which is assigned to the respective line and by means of which the substance and the propellant are injected into the boiler. The substance quantity to be conducted to the respective nozzle is set in the distributor. The substance and the propellant are combined upstream of the branch-off with the respective line.

This application claims priority of European Patent Application No. 08 011 324.4 filed Jun. 23, 2008, the disclosure of which is incorporated herein by reference in its entirety.

The present invention relates to a method for injecting a substance into a boiler of a garbage incineration plant, and to an apparatus for carrying out the method.

Generic methods are used, for example, for the selective non-catalytic reduction (SNCR) of gaseous nitrogen oxides (NO_(x)), such as occur, for example, in garbage incineration. Such SNCR methods are described, for example, in DE-A-4139862 and in WO 2006/053281.

In the SNCR methods, conventionally, reducing agents are injected in aqueous solution (for example, ammonia water or urea) or in gaseous form (for example, ammonia) into the hot smoke gases flowing through the boiler, in order to reduce nitrogen oxides into molecular nitrogen according to the following equations.

4NO+4NH₃+O₂−>4N₂+6H₂O

2NO₂+4NH₃+O₂−>3N₂+6H₂O

The optimal temperature range in this case lies approximately between 850° C. and 950° C., depending on the composition of the smoke gas. The corresponding region of the boiler is also designated as the afterburning chamber.

Above the optimal temperature range, ammonia is increasingly oxidized, nitrogen oxides occurring, which results in an undesirable extra consumption of ammonia.

Below said temperature range, only an inadequate reduction of nitrogen oxides takes place.

In order to remove nitrogen oxides efficiently from the smoke gas, it is therefore necessary to inject ammonia into the boiler primarily where said optimal temperature range is present.

Accordingly, for example, WO 91/17814 discloses a method whereby an agent for the reduction of pollutants is injected into a smoke gas passage at a temperature at which the pollutants are effectively reduced. In this case, the nozzles are arranged in different zones distributed over the height.

Alternatively to this, DE-C-3722523 proposes a nozzle arrangement on a vertically displaceable lance rack, via which the location for the supply of ammonia can be adapted.

However, a further obstacle in the way of an efficient reduction of nitrogen oxides is that the temperature at the individual points in the boiler is usually non-uniform and is subject to pronounced time fluctuations.

In this regard, DE-A-4434943 discloses a method in which the temperature level optimal for reduction in the boiler is determined by temperature sensors, and the nozzles for supplying the reducing agent are oriented correspondingly.

In practice, however, an optimal reduction in the nitrogen oxides is possible only when the quantity of reducing agent to be supplied can be set individually for each nozzle, in order to take the local temperature differences in the boiler into account.

In this regard, DE 20 2006 013 152 discloses a line arrangement for a plurality of atomizer lances on a boiler, through which line arrangement an additive is supplied to the boiler. In this case, the supply quantity of the additive is set for each atomizer lance in a distributor.

All the nozzles or lances of the above-listed documents of the prior art are what are known as two-component nozzles or two-component lances.

Thus, for example, in the arrangement described in DE 20 2006 013 152, the additive and the pressure medium are intermixed, immediately before being supplied to the boiler, in a connection piece assigned to the atomizer lance, thus allowing a mutually independent metering of the propellant and of the additive.

Since each individual two-component lance or two-component nozzle has a specific supply for the additive and a specific supply for the compressed air, however, they have a relatively complicated design. Moreover, two-component lances or two-component nozzles possess relatively large dimensions, thus making it difficult to install them in the boiler of an incineration plant.

If, as is customary in the SNCR methods, ammonia is injected, in the systems based on two-component nozzles the ammonia is present, as a rule, in a highly diluted solution in the form of ammonia water, in order to ensure a uniform distribution of the ammonia to the individual two-component nozzles. For this purpose, softened water is generally used, in order to minimize lime deposits in the nozzle or in the liquid line leading to the latter.

As well as the high outlay involved in the central dilution of ammonia with softened water, systems based on two-component nozzles often have the disadvantage that control is sluggish, that is to say temperature changes in time in the boiler can be reacted to only after pronounced delay. In particular, as a rule, the known systems do not make it possible to react appropriately to brief temperature peaks in the boiler.

The object of the present invention, therefore, is to make available a simple method for injecting a substance, in particular a reducing agent for the reduction of nitrogen oxides, into a boiler of a garbage incineration plant, which method makes it possible to set the desired substance quantity in a directed manner for individual injection points and to adapt this setting quickly.

In particular, the method is to make it possible, at as low outlay as possible in terms of apparatus, in SNCR methods to ensure an optimal reduction of the nitrogen oxides, along with a minimal consumption of reducing agent, while taking into account both the local temperature differences and the temperature changes in time in the boiler.

The object is achieved, according to the invention, by means of the method of claim 1. Advantageous embodiments are listed in the dependent claims.

According to claim 1, the substance to be injected is conducted, starting from at least one distributor, via lines which branch off from the distributor, in each case to a nozzle assigned to the respective line. The substance quantity to be conducted to the respective nozzle is set in the distributor.

Since, according to the invention, the substance to be injected and the propellant are combined before the respective line branches off from the distributor, a mixture containing the substance and the propellant is present before the branch-off. Consequently, structurally simple and robust one-component nozzles can be used, such as are described, for example, in EP-A-0 364 712, the content of which is hereby incorporated by reference.

It has been assumed hitherto that a setting of the substance quantity to be injected can be implemented for each individual nozzle only by using two-component nozzles.

It was found, then, surprisingly, that it is possible, even using one-component nozzles, to set individually for each nozzle the substance quantity which is to be injected.

The method of the present invention is used primarily for the abovementioned SNCR methods. As a rule, the substance to be injected is therefore a reducing agent for the reduction of nitrogen oxides, in particular ammonia or urea.

The invention makes it possible for the quantity of reducing agent to be injected to be set individually for each nozzle as a function of the temperature profile prevailing in each case in the boiler. What can thereby be ensured is that the reducing agent is injected, in accordance with the main emphasis, where the temperature range optimal for reduction is present. The result of this is that, in the case of a high reduction rate of the nitrogen oxides, the drift and therefore too the consumption of reducing agents are minimized, this resulting, overall, in a highly economical and ecological type of operation.

According to one embodiment of the invention, in the event of a change in the setting of the substance quantity to be conducted to the respective nozzle, this change is carried out in an infinitely variable manner, that is to say continuously.

In general, the nozzles are arranged in a plurality of, preferably at least three horizontal nozzle planes (hence oriented, as a rule, transverse to the direction of flow of the smoke gas). According to the above embodiment of the method, the substance quantity to be injected can be displaced infinitely variably from a first nozzle plane to a further nozzle plane, that is to say it is reduced infinitely variably in a first horizontal nozzle plane and is simultaneously raised infinitely variably to a further horizontal nozzle plane. This makes it possible, in the case of SNCR methods, to react optimally to temperature fluctuations in time, which is not possible in the case of an abrupt nozzle plane changeover and the undefined transitional states of this type occurring at the same time.

The invention makes it possible, further, to carry out said infinitely variable change or displacement very quickly. This made possible, inter alia, in that, in the method according to the invention, a central dilution of the substance, such as is, carried out, for example, in the SNCR methods based on two-component nozzles, is not necessary. The present invention thus makes it possible to react appropriately even to brief temperature changes.

As a rule, in this case, the overall quantity of reducing agent to be injected is regulated as a function of the nitrogen oxide content, present downstream of the boiler, in the smoke gas. For this purpose, means for determining the nitrogen oxide content are arranged downstream of the boiler. The nitrogen oxide content determined in this case is compared with a predetermined desired value in order to regulate the required overall quantity of reducing agent.

In order to ensure as high a nitrogen oxide reduction rate as possible, along with a minimal consumption of reducing agent, the quantity of reducing agent to be conducted to the respective nozzle is regulated, as a rule, as a function of the temperature profile in the boiler. It can thus be ensured that the reducing agent is injected, as the main emphasis, where the temperature range optimal for reduction prevails.

To determine the temperature profile, in principle, any temperature measuring apparatus suitable for this purpose may be considered. Appropriate temperature measuring apparatuses are known to a person skilled in the art. Particularly preferably, the temperature measuring apparatuses used are what are known as radiation pyrometers which allow an exact measurement of the gas temperature.

The temperature gradient falling in the direction of flow of the smoke gas can be determined by means of an algorithm known to a person skilled in the art. Thus, by the temperature at a defined point being determined, the temperature profile prevailing in the boiler can be determined in the direction of flow.

Moreover, by the temperature at a further point being determined, it is possible also to determine the temperature profile transversely to the direction of flow via interpolation between the two temperature points. The determination of the temperature at two different points thus makes it possible to determine the overall temperature profile in two dimensions. Accordingly, the temperature profile in the boiler is determined particularly preferably by means of at least two temperature measuring apparatuses. It is conceivable, for example, that at least two temperature measuring apparatuses are provided per distributor.

The quantity of reducing agent locally required depends, moreover, on the local flow velocity, since more reducing agent is required in the case of high flow velocity than in the case of a low flow velocity.

In this regard, it is conceivable that the quantity of reducing agent to be conducted to the respective nozzle is additionally regulated as a function of the flow velocity distribution of the smoke gas in the boiler. In this case, as a rule, it is sufficient to determine the flow velocity once only.

According to the invention, the setting of the reducing agent quantity to be conducted to the individual nozzles takes place in the distributor. In concrete terms, the metered reducing agent is supplied to the propellant stream, specifically, according to the invention, upstream of the branch-off of the line leading to the respective nozzle. Thus, upstream of the branch-off, a mixture containing the propellant and the reducing agent is therefore present.

Furthermore, as a rule, a distribution of the propellant quantity takes place. In this case, in general, a first propellant quantity is set for the nozzles acted upon by the reducing agent, while, for the nozzles not acted upon by the reducing agent, a second propellant quantity is set for cooling said nozzles.

In this case, for distribution, the means known to a person skilled in the art which are suitable for the corresponding purposes may be considered. For example, the quantity of reducing agent or of propellant may be set by means of regulating valves.

The reducing agent is preferably present in aqueous solution. In the case of dissolved ammonia, ammonia water is therefore referred to. Preferably, for example, a commercially available ammonia solution is used, for example a 25% ammonia solution. However, any other solution may also be envisaged which releases ammonia at temperatures prevailing in the boiler, for example a solution containing an ammonium salt, such as ammonium carbonate, ammonium formate and/or ammonium oxalate.

The propellant used is preferably air or steam.

If the propellant used is, for example, air, the ammonia water to be injected is atomized into droplets when being fed into the air stream and immediately thereafter is distributed to the individual nozzles or to the respective lines leading to the nozzles. Any coalescing droplets are atomized again in the nozzle through which the flow passes at sound velocity.

In the nozzles acted upon by the ammonia water, as a rule, a nozzle admission pressure of approximately 1 to 5 bar is set, so that the ammonia water or the ammonia evaporating out in the boiler can penetrate sufficiently far into the smoke gas stream. Those nozzles which are not in operation, that is to say not acted upon by ammonia water, have a smaller air quantity flowing through them for cooling purposes. Thus, upstream of the branch-off of the lines leading to the respective nozzles, a first air stream is set for the nozzles acted upon by the ammonia water, while a further air stream lower than the first air stream is set for those nozzles which are not in operation.

The lines branching off from the distributor and leading to the respective nozzles are, as a rule, in the form of pipes, such as are known to a person skilled in the art for the corresponding purposes. The nozzles used may, for example, be circular-jet nozzles or fan nozzles, further suitable nozzles being known to a person skilled in the art. In particular, an alternating arrangement of circular-jet nozzles with fan nozzles may be envisaged.

The invention relates, further, to an apparatus for carrying out the method described above. The method and the apparatus are explained further with reference to the accompanying figures in which:

FIG. 1 shows a purely diagrammatic illustration of an apparatus according to the invention which is assigned to a boiler of a garbage incineration plant and which comprises overall nine nozzles which can be distributed on three nozzle planes; and

FIG. 2 shows a graph in which the respective temperature range (abscissa) is plotted as a function of the boiler height (ordinate) and the resulting area of temperature distribution in the boiler is depicted, overlaid with the optimal temperature range of the SCNR method, illustrated by dark hatching.

According to FIG. 1, the apparatus 2 is assigned to a boiler 4 of a garbage incineration plant, through which incineration smoke gas flows. The direction of flow of the smoke gas is illustrated in this case by parallel arrows 6.

The apparatus 2 comprises a distributor 8. The latter has issuing into it a supply line 10 for supplying a substance, in particular a reducing agent for the reduction in nitrogen oxides, and a supply line 12 for supplying a propellant. Overall nine lines 14 a-i branch off from the distributor 8, of which three lines 14 a, 14 b, 14 c lead in each case to a nozzle 16 a, 16 b and 16 c of a first horizontal nozzle plane, three lines 14 d, 14 e, 14 f lead in each case to a nozzle 16 d, 16 e and 16 f of a second horizontal nozzle plane with three lines 14 g, 14 h, 14 i lead in each case to a nozzle 16 g, 16 h and 16 i of a third horizontal nozzle plane. In this case, in each case, three nozzles 14 a, 14 d, 14 g and 14 b, 14 e, 14 h and 14 c, 14 f, 14 i respectively are arranged one above the other and therefore in each case in a vertical nozzle plane.

Moreover, the distributor is assigned two temperature measuring apparatuses 18 a, 18 b, by means of which the temperature profile prevailing in the boiler 4 is determined continuously in two dimensions, as described above.

If the apparatus is used in an SNCR method it is additionally assigned (not shown), as a rule, means for determining the nitrogen oxide content present downstream of the boiler, by which means the overall quantity of the reducing agent to be injected is regulated.

As a function of the temperature profile, the quantity of reducing agent to be conducted to the respective nozzle 16 a-i set in the distributor 2 and is combined with the propellant. The propellant stream containing the metered reducing agent quantity is subsequently conducted via the respective line 14 a-i to the nozzle 16 a-i at the desired injection point and is injected into the boiler 4 there. The nozzles not acted upon by the reducing agent are cooled by means of a smaller propellant quantity likewise set in the distributor 8.

The reducing agents used in the SNCR method described are preferably ammonia water which is supplied to a propellant stream consisting of air or steam. The distribution of the ammonia water in this case takes place in such a way that it is injected in the temperature range of 850 to 950° C. optimal for the reduction of nitrogen oxides.

This concept is further illustrated in FIG. 2, according to which the temperature distribution of conventional boilers ranges, as a rule, from approximately 1000 to 1100° C. in the lowermost region of the boiler, that is to say at approximately 2 to 4 m above the grate, to approximately 700 to 900° C. in the upper-most region of the boiler, that is to say at approximately 20 to 40 m above the grate. The region over-lapping with the optimal temperature range in the SNCR method, illustrated by dark hatching, corresponds to that region of the boiler in which the nozzles distributed on the three horizontal nozzle planes should be in operation for an optimal reduction of the nitrogen oxides.

The position of these horizontal nozzle planes in the boiler is reproduced in FIG. 2 by dashed arrows. In this case, the nozzle planes are arranged at irregular distances from one another. Depending on the boiler, however, a regular arrangement of the nozzle planes may also be envisaged.

The graph for a corresponding boiler thus gives information on the arrangement of the nozzle planes which is to be selected in order to ensure injection in the preferred temperature range.

EXAMPLE

A concrete exemplary embodiment of the metering of ammonia water for an SNCR method is reproduced in table 1. According to this exemplary embodiment, the boiler is subdivided into four boiler sectors, to be precise a boiler sector at back left (HL), a boiler sector at back right (HR), a boiler sector at front left (VL) and a boiler sector at front right (VR). The nozzles are arranged in the boiler sectors on three horizontal nozzle planes.

For each individual boiler sector, the temperature is determined at a predetermined point by means of a radiation pyrometer. By means of multiplication by an appropriate factor indicated in table 1, the temperature of the individual nozzle planes in the boiler sectors can be determined on the basis of the respective measured temperature.

10 kg of ammonia water (25% ammonia solution)/h are injected per boiler sector. The apportionment of the ammonia water to the individual nozzle planes takes place in this case as a function of the calculated temperature profile, specifically in such a way that ammonia water is injected, as the main emphasis, where the temperature range optimal for the reduction of nitrogen oxide is present.

As is clear, further, from table 1, a propellant/cooling air quantity of 100 kg/h is set for the nozzles acted upon by ammonia water, while, for the nozzles not acted upon by ammonia water, a reduced propellant/cooling air quantity of 50 kg/h is set for cooling said nozzles.

TABLE 1 HL HR VL VR Total Radiation pyrometer ° C. 910 880 850 860 indication (T) Temperature on 1st plane ° C. 1010 977 944 955 (calculated 1.11*T) Temperature on 2nd plane ° C. 946 915 884 894 (calculated 1.04*T) Temperature on 3rd plane ° C. 883 854 825 834 (calculated 0.97*T) Metering of 25% ammonia kg/h 0.0 0.0 0.3 0.0 water on 1st plane Metering of 25% ammonia kg/h 0.0 1.4 9.7 7.3 40 water on 2nd plane Metering of 25% ammonia kg/h 10.0 8.6 0.0 2.7 water on 3rd plane Propellant/cooling air kg/h 50 50 100 50 on 1st plane Propellant/cooling air kg/h 50 100 100 100 950 on 2nd plane Propellant/cooling air kg/h 100 100 50 100 on 3rd plane 

1. A method for injecting a substance into a boiler (4) of a garbage incineration plant, using a gaseous propellant, the substance being conducted, starting from at least one distributor (8), via lines (14 a-i) which branch off from the distributor (8), in each case to a nozzle (16 a-i) which is assigned to the respective line (14 a-i) and by means of which the substance and the propellant are injected into the boiler (4), and the substance quantity to be conducted to the respective nozzle (16 a-i) being set in the distributor (8), wherein the substance and the propellant are combined upstream of the branch-off with the respective line (14 a-i).
 2. The method as claimed in claim 1, wherein, in the event of a change in the setting of the substance quantity to be conducted to the respective nozzle (16 a-i), this change is carried out infinitely variably.
 3. The method as claimed in claim 1, wherein the substance is a reducing agent for the reduction of nitrogen oxides, in particular ammonia or urea.
 4. The method as claimed in claim 3, wherein the overall quantity of reducing agent to be injected is regulated as a function of the nitrogen oxide content, present downstream of the boiler (4), in the smoke gas.
 5. The method as claimed in claim 3, wherein the quantity of reducing agent to be conducted to the respective nozzle (16 a-i) is regulated as a function of the temperature profile in the boiler (4).
 6. The method as claimed in claim 5, wherein the temperature profile in the boiler is determined by means of at least two temperature measuring apparatuses (18 a, b).
 7. The method as claimed in claim 6, wherein the temperature measuring apparatuses (18 a, b) used are radiation pyrometers.
 8. The method as claimed in claim 1, wherein the propellant used is air or steam.
 9. An apparatus for carrying out the method as claimed in claim 1, comprising at least one distributor (8), from which lines (14 a-i) branch off which lead in each case to a nozzle (16 a-i) for injecting the substance and the propellant into the boiler (4), the distributor (8) being assigned means for setting the substance quantity to be conducted to the respective nozzle (16 a-i), wherein the apparatus comprises means for combining the substance and the propellant upstream of the branch-off of the respective line (14 a-i) from the distributor (8). 